1. Field of the Invention
Embodiments of the invention generally relate to medical devices that measure respiration parameters based on transthoracic impedance signals. More particularly, but not by way of limitation, one or more embodiments of the invention enable a method and apparatus that converts transthoracic impedance signals into a time series of digital values which is filtered with morphological operators to divide the signal into a respiratory component and a cardiac component and further extract metrics such as respiratory rate, inspiration/expiration (I/E ratio), tidal volume and minute ventilation.
2. Description of the Related Art
There are no known methods or apparatus that can both robustly and efficiently extract respiration information from the transthoracic impedance signal. All known devices that attempt to extract respiration information from impedance signals rely on specially designed finite-impulse response (FIR) filters to remove the cardiac component of the impedance signal while attempting to retain the respiration component of the signal. Although the respiration signal and cardiac signal have different dominant frequencies, the frequency spectrums of these signals do overlap. Therefore, the morphology of the respiration signal extracted from the impedance signal after applying these filters is often distorted. External noises in the impedance signal, such as baseline wander and impulse artifacts, can negatively affect the filter performance. In addition, the complex filters described require special hardware or firmware design, which adds complexity to the implantable device and their operation require more computation power.
Morphological operators have been widely used in 2D image processing for noise removal, and have shown to have better edge preservation performance than other linear or nonlinear filters. The morphological operators have very high computation efficiency, and can be implemented in hardware platform, thus they are particularly suitable for application in low-power devices. However, the application of morphological operators in 1D signal processing, in particular biomedical signal processing has been limited. Morphological operators were used to implement a peak-valley extractor for QRS complex detection in ECG signals. Another morphological approach was developed to detect QRS complexes and remove baseline wander in neonatal ECG signals. Such approach was disclosed in U.S. Pat. No. 5,817,133 issued to Houben, for discriminating P waves from far-field R waves in an implantable pacemaker. However, there are no known solutions that utilize morphological operators to determine respiration parameters from a transthoracic impedance signal.